This invention relates to an alignment system used to aid the alignment of an object lowered from the ocean surface to a predetermined location on a subsea structure. More particularly, the invention pertains to a method and apparatus for accomplishing the necessary alignment without the use of guidelines.
The development of offshore oil fields frequently involves the construction of fixed subsea facilities for use in conducting drilling, completion and production operations. Fixed subsea facilities are especially useful in oil fields located in deep water where conducting such operations from fixed or floating surface structures would be either prohibitively expensive or technically unfeasible. The installation, operation and maintenance of subsea facilities requires that various types of equipment be lowered from the surface of the body of water to a precise location on the subsea structure. In order to accomplish this, means must be provided for properly aligning the lowered equipment with the target area on the structure.
The most widely employed method of accomplishing this alignment is by the use of guidelines. In a typical guideline system a base is mounted on the subsea structure at the target location. One or more upright guideposts are attached to the base. A tensioned wire rope guideline is connected to the top of each guidepost and extends upwardly to the surface of the body of water. The equipment being lowered is attached to a guide frame which is lowered down the guidelines until it engages the guideposts. In this manner the equipment is directed to the desired position on the subsea structure. See, for example, the guideline system disclosed in U.S. Pat. No. 3,050,139 issued to Hayes (1962). Problems which can occur with guideline systems include breakage and entangling of the lines. Furthermore, when an installation is temporarily abandoned and the guidelines are retrieved, future operations require that the connection between the surface and the subsea facility be re-established by divers or by other means. Considering that the difficulties associated with guideline systems increase as water depth increases, their use in very deep water is often impractical.
Several alignment methods have been developed which eliminate the need for guidelines. Divers have been used to perform some subsea alignment operations, but their use is not feasible in deep water. Visual observation of the equipment during positioning through subsea TV cameras has also been used to accomplish the necessary alignment. See, for example, U.S. Pat. No. 3,265,130 to Watkins (1966). TV cameras, however, cannot be relied on for all alignment operations as the sea may be too murky to permit viewing of the operation.
Another guidelinesless alignment method is disclosed in U.S. Pat. No. 3,545,539 to Manning (1970). Manning discloses an alignment system comprising a foundation unit having a plurality of stabbing sleeves attached thereto and a satellite body having a plurality of vertical stabbing columns depending therefrom. One of the stabbing columns is longer then the others. This longer stabbing column is partially inserted into its corresponding stabbing sleeve on the foundation unit to provide an axis around which the satellite body may be rotated until the other stabbing columns are vertically aligned with their respective stabbing sleeves. The satellite body is then lowered into its final position. The alignment system disclosed by Manning is not capable of precise rotational alignment without the aid of a manned submersible vessel. When the longer stabbing column has been partially inserted into its stabbing sleeve, there is no way of knowing, absent visual observation, whether the other stabbing columns and stabbing sleeves are properly aligned. Thus, a manned submersible vessel or other means of visual observation is required to properly position the satellite body. The propulsion system attached to the satellite body will cause over-rotation or under-rotation unless directly controlled by visual observation.
In another guidelineless alignment method, a remote guidance system uses sonar or acoustic signals and TV cameras to locate the lowered equipment with respect to the sea floor target. Once the location of the equipment is determined, the equipment is moved laterally and rotated by a propulsion system to attain its proper position above the target. The propulsion system may be combined with the guidance system to automatically make position corrections. See, for example, the remote guidance system disclosed in U.S. Pat. No. 4,167,215 to Thorne (1979). Such systems, however, are often inadequate due to their limited precision. Consequently, once a coarse adjustment has been effected using the remote guidance system, an auxilliary method is required for precise final alignment.
Thus, it can be seen from the above that a need exists for a remote, guidelineless alignment method which is capable of precise alignment.